1. Field of the Invention
The present invention relates to combining light beams using diffractive beam combining. More specifically, the invention relates to combining a plurality of laser beams in multiple stages, where in each stage, a diffractive optical element operating as a beam combiner diffracts a low power sample of combined beams for use in active phasing.
2. Description of Related Art
High power lasers have many possible applications. In a military application, sufficient energy focused in a laser beam can provide an effective defensive countermeasure against ballistic projectiles. In a commercial application, a high power laser can be used to weld metal components that are too thick to be welded by conventional methods. To improve the effectiveness of the laser in any of these applications, the power transmitted by the beam may be enhanced by focusing the beam to its far-field diffraction limit, i.e., into as small an area as theoretically possible. A laser beam focused to this theoretical limit is said to be diffraction limited. Generally speaking, advancement of the art of high power lasers is impeded by physical limitations encountered when attempting to achieve this limit.
Lasers by their nature are ideally diffraction limited, such that the smallest possible area of focus is limited by the product of the focal distance and the diffraction limited angle, which is the wavelength of the light divided by the aperture width. Thus, the larger the aperture, the tighter the focus. However, there are practical limitations on the size of an aperture that can be designed for any optical apparatus. Imperfections in the optics may cause degradations in the laser wavefront that affect the focus, and in high power applications, thermal variations contribute to the degradation. This limits the designer's ability to focus the beam, resulting in a focal spot somewhat greater than 1.0 times the diffraction limit (1.0×DL). Practically speaking, the designer's goal is to achieve a near-diffraction-limited laser (i.e., one that approaches 1.0×DL) that operates at as high a power level as possible.
At present, the most advanced near-diffraction-limited laser beams cannot deliver enough power per unit area to serve effectively in high-power applications. In one case, an optimized beam can deliver a 3 kW beam having a diffraction limit of nearly 1.0. In another case, an optimized beam can deliver a 10 to 12 kW beam that is about 1.5 times diffraction limited. An objective of ongoing research in this field is to design a laser generator that can deliver 100 kW or more in a near-diffraction-limited beam.
One method for increasing the power deliverable by lasers is to combine the power of many coherently phased beams of a common wavelength by arranging a plurality of optical fiber emitters in a two-dimensional array. A beam splitter may be placed at the output of the array to sample the constituent beams. Each of the sampled beams is directed to a phase sensor, and the measured error signals are provided to phase modulators in each beam to ensure all the beams have equal phase. However, even in the most tightly packed array, the ‘fill factor’ of the composite beam (ratio of the composite beam power to a beam that uniformly fills the entire array aperture and has equal peak intensity) is only about 70%, due to voids that occur between fibers and also to the Gaussian shape of each beam. The end result is a reduction in brightness by the fill factor ratio—the focused composite beam has a central peak intensity equal to the fill factor ratio times the maximum intensity possible with an ideal uniform beam, with the remaining power appearing in widely separated side lobes. In other words the composite beam has a shape dramatically different than that of the constituent beams, and as a result the composite does not focus as well as the constituents.
Another known method for combining beams is spectral combining, in which many incoherent beams, i.e. beams of different wavelengths, are superimposed. The beams are transmitted through a prism or grating that aligns the beams along a common path, creating, in essence, a singular beam of multiple colors. Thus the composite beam has a shape that is substantially identical to that of the constituent beams. While this technique therefore eliminates the fill factor problem associated with the two-dimensional array, other problems arise from using multiple wavelengths. For one, the complexity of the system increases as each wavelength requires a different oscillator. Furthermore, the propagation angle of each wavelength must be precisely adjusted such that its incidence on the grating is exact, otherwise the beams will misalign. More importantly, each wavelength may behave differently as the beam propagates through various media. Atmospheric absorption is a function of wavelength, therefore a spectrally combined beam directed through air is more susceptible to energy loss than a single-wavelength selected for optimal transmission efficiency. Spectral combining has been proposed, for example, in U.S. Pat. No. 6,697,192, U.S. Pat. No. 6,327,292, U.S. Pat. No. 6,208,679, and U.S. Pat. No. 6,192,062.
Another proposed technique for increasing the power in a laser beam is to (coherently) combine, by constructive interference, a plurality of beams into a single coherent beam. This technique, known as coherent diffractive beam combining, is the subject of co-pending U.S. patent application Ser. No. 11/361,352 filed Feb. 24, 2006, which is incorporated by reference herein as though set forth in full. In general, the co-pending application teaches generating a plurality of input beams, all having a common wavelength, using a master oscillator. Each beam is individually amplified and transmitted through a fiber emitter, and the emitter outputs are combined into a single output beam using a diffractive optical element (DOE). The technique includes a means for actively controlling the phases of the multiple beams using feedback to optimize the efficiency of the beam combination. This may be accomplished by coupling a phase modulator to each input beam, and by coupling a phase detector to a sampling of the output beam. The sampling is obtained by placing a transmissive beam splitter in the output path that reflects a low power portion of the output to the phase detector. Using electronics, correction signals based on phase deviations detected at the output are fed back to the modulators. An exemplary means for effecting active phase control in this fashion is disclosed in U.S. Pat. No. 6,708,003, which is also fully incorporated herein by reference. Another active phase detection and control method has been demonstrated by T. M. Shay et al., Proceedings of the SPIE, Vol. 5550, pp. 313-319 (2004), which is also fully incorporated herein by reference. An advantage of this approach is that, similar to SBC, the combined output beam has a shape that is substantially identical to the composite beams and therefore eliminates the fill factor reduction in the intensity of the focused coherent output beam. However, disadvantages occur when sampling the phases of the high power combined output beams. A high power beam passing through a transmissive beam splitter causes thermal distortion that affects the phase measurement accuracy and focusability of the output beam. Also in this method, a single detector is used to measure the phases of all the constituent beams. For a very large number of combined beams the accuracy of phase measurement becomes more difficult with a single detector.